Is vs == in Python: Key Differences and When to Use Each

In Python, is checks if two variables point to the exact same object in memory, while == checks if the values of two objects are equal. Use is for identity comparison and == for value equality.

Quick Comparison

Here is a quick table showing the main differences between is and == in Python.

Aspect	is	==
Purpose	Checks if two variables refer to the same object	Checks if two objects have equal values
Type of comparison	Identity comparison	Equality comparison
Works with	All objects (memory addresses)	Objects that define equality (__eq__ method)
Result type	Boolean (True or False)	Boolean (True or False)
Example use case	Check if two variables are the same object	Check if two objects have the same content
Common mistake	Not for comparing values	Does not check if objects are identical

Key Differences

The is operator compares whether two variables point to the exact same object in memory. This means it checks the identity of the objects, not their content. For example, two different lists with the same values will be == equal but not is equal.

On the other hand, == compares the values inside the objects. It calls the __eq__ method of the objects to determine if their contents are equal. This is why two separate objects can be == equal but not is equal.

Using is is common when checking if a variable is None because None is a singleton object in Python. For value comparisons, always use ==.

Code Comparison

python

a = [1, 2, 3]
b = a
c = [1, 2, 3]

print(a is b)  # True because b points to the same object as a
print(a == b)  # True because values are the same

print(a is c)  # False because c is a different object
print(a == c)  # True because values are the same

Output

True
True
False
True

'==' Equivalent

python

x = [4, 5, 6]
y = [4, 5, 6]

print(x == y)  # True because values are equal
print(x is y)  # False because different objects

z = x
print(z is x)  # True because z points to the same object
print(z == x)  # True because values are equal

Output

True
False
True
True

When to Use Which

Choose is when you want to check if two variables point to the exact same object, such as when comparing to None or singletons. Choose == when you want to check if two objects have the same value or content, like comparing strings, lists, or custom objects.

Using is for value comparison can lead to bugs because two objects can have the same value but be different objects. Using == for identity checks can also be incorrect because it only compares values, not object identity.

Key Takeaways

is checks if two variables refer to the same object in memory.

== checks if two objects have equal values.

Use is for identity checks like comparing to None.

Use == for comparing values of objects.

Confusing is and == can cause subtle bugs.